There are some electricity storage devices whose internal resistances increase and thus charging and discharging capacities are lowered, with decreasing temperatures of the electricity storage devices. For example, as for the internal resistance with respect to a battery temperature of a typical lithium (Li) ion battery, the internal resistance when the battery is used at 10° C. is about 2.5 times greater than the internal resistance when the battery is used under an optimum temperature condition (25° C.). For this reason, when charging or discharging is performed at low temperatures, such a problem occurs that a voltage greatly decreases or increases to fall outside an operable battery voltage range, inhibiting a battery from outputting a desired amount of charging or discharging electricity. Two solutions to the problem with an inability to output a desired amount of charging or discharging electricity at low temperatures are as follows.
The first solution is a method of setting a temperature of an electricity storage device to a preset temperature by warming up a neighborhood of the electricity storage device. For example, Chevrolet Volt plug-in hybrid vehicles are equipped with a refrigerant circuit for a battery. This refrigerant circuit functions to not only cool down the battery, but also heat the battery when an operation temperature decreases below an optimum range.
In addition, the second solution disclosed in Patent Document 1 is a method of forcibly performing charging or discharging within a predetermined charging range of an electricity storage device. In this second solution, heat generating as a result of the charging or discharging is used to control a temperature of the battery so as to become a preset temperature. When a temperature of the battery is lower than the preset temperature, charging or discharging is forcibly performed to feed current to the battery, and this leads to generation of heat from the internal resistance of the battery.